1. Field of the Invention
Examples of the subject matter disclosed herein generally relate to apparatus, methods and systems for arresting cracks in ductile steel pipelines.
2. Background
Modern gas pipelines operate at high pressures that result in circumferential (hoop) stresses that are up to 80% of the steel's specified minimum yield strength. Crack arrestors are used in these pipelines to stop the unrestricted axial fractures that initiate due to damage from corrosion and construction equipment such as backhoes. Some previous forms of crack arrestors increase the hoop resistance of the pipeline steel to stop the crack from further propagation.
If the crack, or axial fracture, encounters an abrupt change in hoop stiffness or resistance at the crack arrestor, the crack may change direction from axial to circumferential. Since the tear or crack now starts to propogate around the circumference of the pipe, this occurrence is commonly referred to as “ring-out”. The circumferential crack may cause the pipe to separate into two parts with the still-pressurized parts acting with an unrestrained jetting action. The unrestrained jetting action may eject the buried pipe segments out of its trench or cause an above ground pipe to whip violently.
U.S. Pat. No. 3,349,807 to Penman discloses particular means for limiting the extent to which high pressure compressible fluid conduits may rip should a crack form in a conduit. A circumferential steel strap is used which has a length slightly less than the perimeter of the conduit and tightening the band onto the conduit by two flanges formed on the respective ends of the bands which are pulled together by a series of bolts. Pennman '807 fails to disclosue maintaining circumferential tension in a composite material wherein the composite material resides around a metal pipe; and maintaining compressive circumferential residual stress on the pipe with the composite material. Penman '807 fails to teach disposing a composite material around a length of metal pipe; wherein the composite material comprises non-metallic fibers and resin; wherein the disposed composite material comprises fibers aligned to the pipe circumference; applying radial force inside the pipe, the applied radial force yielding the pipe past the elastic limit of the pipe and creating circumferential tension on at least a portion of the composite material; wherein the radial force inside the pipe is applied with hydraulic or mechanical means or a combination of both as is used by a pipe expander; and wherein at least a portion of the composite material remains in circumferential tension upon release of the applied radial force. Pennman '807 fails to disclose maintaining circumferential tension in a composite material where the composite material resides around a metal pipe; and maintaining compressive circumferential residual stress on the pipe with the use of the composite material.
U.S. Pat. No. 4,559,974 to Fawley discloses a crack arrester for stopping a propagating ductile fracture in an object such as a pipe that comprises a band defined by a plurality of continuous, high tensile strength, nonmetallic fibers which are wound around the object in intersecting relationship with the direction of propagation and are encapsulated in a resin matrix which is cured. Fawley '974 fails to teach applying a radial force inside the pipe, wherein the radial force inside the pipe is applied with hydraulic or mechanical means or a combination of both as is used by a pipe expander; the applied radial force yielding the pipe past the elastic limit of the pipe and creating circumferential tension in at least a portion of the composite material; and wherein at least a portion of the composite material remains in circumferential tension upon release of the applied radial force. Fawley fails to teach or suggest maintaining circumferential tension in a composite material. Fawley also fails to teach or suggest maintaining a residual stress in the pipe. Fawley '974 does not disclose use of a radial force or over-strain to establish an initial compressive residual stress in the metal as disclosed above.
U.S. Pat. No. 6,435,218 to Hillenbrand et al. discloses pipe with crack stopper feature comprising a steel pipe which includes an outer corrosive-preventive protective sheath, which has a surface layer of plastic material, and a crack stopper zone extending over a section of the pipe. The crack stopper zone includes a bandage which is wrapped around the pipe section and made of high-strength fiber material imbedded in a compacted matrix of thermoplastic and thermosetting plastic. The bandage is comprised of a plurality of wound layers joined to one another via the matrix by welding or gluing. Hillenbrand '218 fails to teach applying a radial force inside the pipe wherein the radial force inside the pipe is applied with hydraulic or mechanical means or a combination of both as is used by a pipe expander; the radial force applied yielding the pipe past the elastic limit of the pipe and creating circumferential tension on at least a portion of the composite material; and wherein at least a portion of the composite material remains in circumferential tension upon release of the applied radial force.
Several patents describe the use of composite materials to make a crack arrestor by simply adding the composite material to the pipeline metal. These patents do not intentionally impose a compressive residual stress on the metal. This is important since I find that the imposed residual stresses on the metal causes the crack to stop its progress, which is the intent of the crack arrestor. A novel point, therefore, is that the crack arrestor if made as I propose should require less composite material in manufacturing. This is because the arresting action does not solely depend on the strength and quantity of the composite materials; the residual compressive stresses in the metal play a major role in arresting the crack by reducing the tensile hoop stresses of the metal in operation.